Forming of a metallic layer onto a substrate bearing a thin conductive layer, usually copper, in an electrolyte environment, is implemented to form conductive lines during VLSI (ultra large scale integrated) circuit fabrication. Such a process is used to fill cavities, such as vias, trenches, or combined structures of both by electrochemical methods, with an overburden film covering the surface of the substrate. It is critical to obtain a uniform final deposit film because the subsequent process step, commonly a planarization step (such as CMP, chemical-mechanical planarization) to remove the excess conductive metal material, requires a high degree of uniformity in order to achieve the equal electrical performance from device to device at the end of production line.
Currently, metallization from electrolyte solutions is also employed in filling TSV (through silicon via) to provide vertical connections to the 3-D package of substrate stacks. In TSV application, via opening has a diameter of a few micrometers or larger, with via depth as deep as several hundreds of micrometers. The dimensions of TSV are orders of magnitude greater than those in a typical dual damascene process. It is a challenge in TSV technology to perform metallization of cavities with such high aspect ratio and depth close to the thickness approaching that of the substrate itself. The deposition rates of metallization systems designed for use in typical dual damascene process, usually a few thousand angstroms per minute, is too low to be efficiently applied in TSV fabrication.
To achieve the void-free and bottom-up gapfill in deep cavities, multiple organic additives are added in the electrolyte solutions to control the local deposition rate. During deposition, these organic components often break down into byproduct species that can alter the desired metallization process. If incorporated into deposited film as impurities, they may act as nuclei for void formation, causing device reliability failure. Therefore, during the deposition process high chemical exchange rate of feeding fresh chemicals and removing break-down byproducts in and near the cavities is needed. In addition, with high aspect ratio, vortex is formed inside the cavities below where steady electrolyte flow passes on top of the cavity openings. Convection hardly happens between the vortex and the main flow, and the transport of fresh chemicals and break-down byproducts between bulk electrolyte solution and cavity bottom is mainly by diffusion. For deep cavity such as TSV, the length for diffusion path is longer, further limiting the chemical exchange within the cavity. Moreover, the slow diffusion process along the long path inside TSV hinders the high deposition rate required by economical manufacturing. The maximum deposition rate by electrochemical methods in a mass-transfer limited case is related to the limiting current density, which is inversely proportional to diffusion double layer thickness for a given electrolyte concentration. The thinner the diffusion double layer, the higher the limiting current density, thus the higher the deposition rate possible. Patent WO/2012/174732, PCT/CN2011/076262 discloses an apparatus and method by using ultra/mega sonic in the substrate metallization to conquer the above issues.
In the plating bath used a piece of ultra/mega sonic device, the wave distribution across the ultra/mega device length is not uniform, which is proved by the power intensity test of acoustic sensor and other optical-acoustic inspection tool. To apply it on the substrate, the acoustic energy dose on each point of the substrate is not the same.
In addition, in the plating bath with an acoustic field, the wave energy lost occurs due to wave propagation absorbed by the bath wall and diffraction around the additives and byproducts. So that the power intensity of acoustic wave in the areas near the acoustic source are different from those far away from the acoustic source. A standing wave formed in two parallel planes maintains the energy within the bath to minimize the energy lost. And the energy transfer only occurs between the node and anti-node within a standing wave. However, the power intensity of wave in its node and anti-node are different, which leads to not uniform acoustic performance across substrate during process. What's more, it is difficult to control the standing wave during the entire process due to the difficulty in adjustment for the parallelism and distance between the surfaces forming standing wave.
With this method; however, a way of controlling uniformity of acoustic energy distribution further the uniformity of plating deposition must be found. And a way of controlling the acoustic field with low energy lost in the plating bath is further required.